In papermaking, for instance, the thickness of paper is measured from a moving paper web. In this measurement, solutions can be used, in which the sensor of the measuring device touches the surface of the paper, or solutions in which the sensor does not touch the surface. Solutions that do not touch the surface include capacitive measurements and optical measurements. The contact of the sensor on the surface being measured may cause surface errors and, therefore, solutions in which the sensor touches the surface of the object being measured are avoided.
Optical measurements utilise chromatic aberration in determining the surface being measured. In this solution, light is focused on the surface through an optical element whose focal distance is in a known manner dependent on the wavelength of light. Light reflected from the surface is collected coaxially to a detector with the same optical element. The detector, which may be a spectrum analyser, analyses the spectrum of the reflected light. The wavelength with which the surface is best in focus is also reflected the most efficiently, and it represents the highest intensity in the spectrum. Because on the basis of the dimensioning of the measuring device, the location of the focal point of this wavelength is known, it helps define the location of the surface. If the surface is determined on both sides of the paper, it is also possible to measure the thickness of the object.
However, there are problems related to the known solution that utilises chromatic aberration. When the object being measured is of a diffuse material, light is reflected not only from the surface but also from inside the object. This causes a measurement error. For instance in measured objects into which light penetrates less, the measurement gives systematically a greater thickness than in measured objects of the same thickness into which light penetrates more. Attempts have been made to correct the measurement error computationally by altering the measuring result according to the quality of the measured objects. However, extensive corrections of measuring results reduce measuring accuracy and reliability, and an object that has not been determined in advance cannot be accurately measured.